The present invention relates to a document illumination apparatus for an electrophotographic copying machine and, more particularly, to a cooling arrangement associated with the document illumination lamp to prevent platen overheating.
In conventional copying machines, an original document to be reproduced is placed or conveyed onto a glass platen located in the object plane. The document is incrementally illuminated by an elongated lamp assembly, either by moving the platen while the lamp is held stationary, or by moving the lamp assembly (lamp reflector and scan mirror) beneath a stationary platen. Alternatively, both the scan lamp and the platen can be held fixed and the document moved across the platen surface in a continuous velocity transport (CVT) mode. In any of these cases, a problem to be anticipated and prevented is a build-up of heat at the platen, the heat being a byproduct of operation of the scanning lamp. The lamps used for document scanning are typically mercury fluorescent or tungsten halogen. With either source, but particularly with the tungsten halogen type of lamp, the heat build-up at the platen may pose a problem with continued operation. With continued usage, the platen may become so hot as to present a safety concern for the machine operator or cause defects in the platen glass.
There are various prior art solutions to the lamp overheating problem. One solution is to provide some kind of cooling to the lamp, using blowers or fans to create a flow of heated air moving into a vent area of the machines. Representative of this type of solution is U.S. Pat. No. 4,143,964 which disclosed placing the illumination lamp at the focal point of a curved apertured reflector, and blowing air through the aperture onto the lamp along its length. U.S. Pat. No. 4,632,539 discloses an apparatus for cooling a fluorescent lamp by causing a flow of cooling air to pass along a conduit extending along the length of the lamp. U.S. Pat No. 3,936,672 discloses a ventilating fan which blows air into a casing surrounding the lamp.
Another design solution to the platen overheating problem is to utilize the principles of heat transfer by placing a heat transfer mechanism in proximity to the lamp to direct heat away from both the lamp and the platen. U.S. Pat. No. 4,411,516 discloses a system for cooling an area surrounding a halogen lamp by forming a heat pipe integral with the reflectors surrounding the lamp. The heat pipe serves to draw heated air out of the area beneath the lamp and the platen.
Another technique for reducing platen overheating is found in U.S. Pat. No. 4,183,656. The heat buildup is moderated by controlling the travel distance of the scanning assembly coupled with periodic interruption in lamp operation. U.S. Pat. No. 4,684,237 discloses a CVT type of system where the illumination assembly, stationary during exposure, is alternately moved to new scan locations after each exposure, thus preventing heat buildup in any given incremental width area of the platen.
Other techniques require a heat-containing or reflecting component to be interposed between the lamp and the platen. Japanese publication 56--62269 (Kiyougora) discloses a transparent filter with a heat-reflecting layer interposed between the lamp and the platen. U.S. Pat. No. 3,936,672 referenced supra, also disclosed that the lamp, partially enclosed by reflector 5, further has a screen 6 which maybe glass or a red filter to reflect infrared rays and prevent them from striking the platen. U.S. Pat. No. 4,411,516 also discloses an embodiment, shown in FIG. 3, wherein a glass plate with an interference film applied to the plate ("hot mirror 28") transmits the visible light from the illumination lamp but reflects the heat rays.
Finally, U.S. Pat. No. 3,712,731 discloses still another method for preventing heat buildup at the platen by interposing a heat shield plate downstream from the illuminated zone.
Each of the foregoing solutions to the platen overheating problem have various drawbacks and disadvantages. Those designs requiring blower motors or specially constructed filters add an additional expense to the system design. Changing the scan distance or scan position requires additional complexities of system control.
The present invention is, therefore, directed to a simple, inexpensive solution to the platen overheating problem described above. It has been found that platen heat buildup can be reduced by enveloping the illumination lamp within a glass tube annulus open at both ends. The glass properties are designed to withstand the heat output generated by the particular illumination lamp used. In the embodiment disclosed, a halogen lamp is used with a PYREX.TM. tube. As the lamp assembly in one embodiment is moved along a document scan path, a portion of the heat generated by the lamp is entrapped between the lamp envelope and the annulus envelope, and passes out of both ends of the annulus into a vent area, along a convection path. Additional heat energy is absorbed by the glass tube and transferred by convection to the air surrounding the glass tube or conducted via heat conduction paths that can exist between the glass tube reflector and underlying scan carriage.
In further embodiments, the basic design is enhanced by creating a positive air flow by means of blowers and duct work. More particularly, the invention is directed towards a document scan/illumination system wherein a document on a document platen is incrementally illuminated by a scan assembly beneath said platen, said scan assembly adapted to reduce in heat buildup at the platen, and including, in combination, an elongated illumination lamp and a glass tube annulus open at both ends encompassing said lamp to create a partially confined interior space between said lamp and annulus. In a specific embodiment a scan carriage is adapted to carry said lamp and annulus beneath the platen in a document scanning mode of operation whereby heat generated by said lamp is at least partially contained within said interior space, and is transferred by convection through the open ends of the annulus. In addition, a portion of the heat generated by the lamp is absorbed by the glass tube and transferred by convection to the air surrounding the glass tube. Additional heat is also transferred via conduction paths that exist between the glass tube, reflector and scan carriage.